6. f4g5 Carlsen accepts the pawn sac. 6... e7e5 The idea with the sac is to control the center with the pawns. Next is d7-d5. 7. g2g4 A subtle move to gain access to the e4-square. 7... f5f4 A forced reaction. 8. e3b6 Moving the bishop to d2 or f2 could also be considered.